1. Field of the Invention
This invention relates to devices relying on magnetic properties and, more particularly, to the fabrication of these devices.
2. Art Background
Devices relying on magnetic properties often require the deposition of a metal film during their fabrication. For example, in the case of magnetic bubble devices, an aluminum alloy is deposited on a silicon dioxide layer that, in turn, overlays the magnetic garnet epilayer. These metallic films are often patterned to produce a desired result in a localized area of the device. In the example of magnetic bubble devices, the aluminum alloy does not cover the entire silicon dioxide layer, but is confined to areas where control functions are performed, e.g., the metal film is patterned to induce bubble nucleation, replication or transfer in a particular area of the magnetic garnet film at a given instant in time.
Since the metallic films utilized are not continuous, but are localized in particular areas of the devices, subsequent deposited layers will not fill in the steps produced by this localization. Thus, continuing the example of magnetic bubble devices, if another silicon dioxide layer is deposited over the aluminum alloy, this silicon dioxide layer will not be planar, but will have depressions in areas where the underlying aluminum alloy is absent.
This non-planar structure, although usually unimportant in semiconductor devices, often becomes significant in devices which rely on magnetic properties. Since magnetization is a three dimensional effect, a film that is not planar experiences magnetic gradients through its cross-section. For example, in the case of magnetic bubble devices if a permalloy alloy is deposited on a stepped silicon dioxide film, this permalloy alloy is similarly non-planar. When magnetic fields are introduced to operate the device various areas of the permalloy strip experience spurious magnetic effects. This results in a degree of device unreliability. (See, for example, W. Strauss, Journal of Applied Physics, 49 1897 (1978).)
Various fabrication schemes have been developed to produce a planar geometry in devices relying on magnetic properties. For example, Yamagishi has described (Third U.S.A.-Japan Computer Conference. Oct. 10th through 12th, 1978, Session 20-3-1) a method of forming a planar geometry on an underlying substrate. This process is schematically illustrated in FIG. 1. The first step, 1 in FIG. 1, includes the deposition of an aluminum alloy, 8, upon the substrate, which is a silicon oxide, 7, coated magnetic epilayer, 9. In the second step, 2, a resist material, 11, is deposited on the alloy which in the third step, 3, is etched in the exposed areas, 12. A layer of silicon monoxide, 14, is then deposited on the substrate thus covering both the resist and the exposed areas of the aluminum alloy. The resist is then lifted off and the planar geometry, 5, is obtained.
A comparison of FIG. 1 which shows the Yamagishi technique and FIG. 2 which shows the steps required in the conventional non-planar production method demonstrates that an additional step involving the deposition of a silicon monoxide layer is required. Obviously, this additional step leads to associated production difficulties and costs. Other proposed methods for fabricating devices with planar geometries require even more processing steps with their associated costs and difficulties. (See, for example, Rose, IEEE Transactions on Magnetics, MAG-12, (6), 618, (1976), and Reekstin, et al, IEEE Transactions on Magnetics, MAG-9, (3), 485 (1973).) Thus, although the desired planar geometries have been achieved, this achievement requires significantly increased processing costs.